As scaling of metal-oxide-semiconductor field-effect-transistors (MOSFETs) continues, the doping concentration, gate oxide thickness, and source/drain (S/D) doping profiles that are required to control short-channel effects become increasingly difficult to meet. The heavy channel doping often required to provide adequate suppression of short-channel effects results in degraded mobility and enhanced junction leakage. In addition, the reduction of gate dielectric thickness for reduced short-channel effects and improved drive current leads to increased direct tunneling gate leakage current and standby power consumption.
One approach to controlling short-channel effects calls for employing a thin silicon film as the MOSFET channel, thereby diminishing or possibly eliminating sub-surface leakage paths. Such a concept is embodied in the thin-body MOSFET, in which the source-to-drain current is restricted to flow in a region near the gate. By not relying on a heavily-doped channel for the suppression of short-channel effects, mobility degradation due to impurity scattering may be reduced, as well as the threshold voltage fluctuation conventionally attributable to the random variation of the number of dopant atoms in the channel region.
However, quality control of the crystalline nature of such thin channels and manufacture of such devices have proven to be challenging. Moreover, the high impedance resulting from thin S/D regions often limits the device performance as device scaling continues.
S/D stressors having a SiGe composition have also been demonstrated as a possible approach not only to controlling short-channel effects due to retarded boron diffusion in SiGe but also improving hole mobility due to compressive strain in the channel. However, arsenic and phosphorus diffusion is enhanced in SiGe and compressive strain can significantly degrade electron mobility. Therefore, such an approach can only offer enhancements to p-MOSFET performance, while employing germanium in n-MOSFET S/D regions can actually exacerbate short-channel effects and diminish device performance.